title: Side-chain effects on the solution-phase conformations and charge photogeneration dynamics of low-bandgap copolymers.
abstract: Solution-phase conformations and charge photogeneration dynamics of a pair of low-bandgap copolymers based on benzo[1,2-b:4,5-b(')]dithiophene (BDT) and thieno[3,4-b]thiophene (TT), differed by the respective carbonyl (-C) and ester (-E) substituents at the TT units, were comparatively investigated by using near-infrared time-resolved absorption (TA) spectroscopy at 25 °C and 120 °C. Steady-state and TA spectroscopic results corroborated by quantum chemical analyses prove that both PBDTTT-C and PBDTTT-E in chlorobenzene solutions are self-aggregated; however, the former bears a relatively higher packing order. Specifically, PBDTTT-C aggregates with more π-π stacked domains, whereas PBDTTT-E does with more random coils interacting strongly at the chain intersections. At 25 °C, the copolymers exhibit comparable exciton lifetimes (~1 ns) and fluorescence quantum yields (~2%), but distinctly different charge photogeneration dynamics: PBDTTT-C on photoexcitation gives rise to a branching ratio of charge separated (CS) over charge transfer (CT) states more than 20% higher than PBDTTT-E does, correlating with their photovoltaic performance. Temperature and excitation-wavelength dependent exciton∕charge dynamics suggest that the CT states localize at the chain intersections that are survivable up to 120 °C, and that the excitons and the CS states inhabit the stretched strands and the also thermally robust orderly stacked domains. The stable self-aggregation structures and the associated primary charge dynamics of the PBDTTT copolymers in solutions are suggested to impact intimately on the morphologies and the charge photogeneration efficiency of the solid-state photoactive layers.
authors.affiliation: Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150001, China.
authors.forename: Ming-Ming
authors.initials: MM
authors.lastname: Huo
authors.forename: Ran
authors.initials: R
authors.lastname: Liang
authors.forename: Ya-Dong
authors.initials: YD
authors.lastname: Xing
authors.forename: Rong
authors.initials: R
authors.lastname: Hu
authors.forename: Ning-Jiu
authors.initials: NJ
authors.lastname: Zhao
authors.forename: Wei
authors.initials: W
authors.lastname: Zhang
authors.forename: Li-Min
authors.initials: LM
authors.lastname: Fu
authors.forename: Xi-Cheng
authors.initials: XC
authors.lastname: Ai
authors.forename: Jian-Ping
authors.initials: JP
authors.lastname: Zhang
authors.forename: Jian-Hui
authors.initials: JH
authors.lastname: Hou